206         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,    1907,   PAET   II. 
lower  than  the  apparent  unconformity  previously  mentioned.  Ac- 
cording to  F.  II.  Knowlton  these  plants  are  Fort  Union  forms,  and  if 
further  study  confirms  this  opinion,  the  Fort  Union  probably  includes 
several  hundred  feet  of  beds  which  are  considered  Laramie  in  tins 
report,  and  the  thickness  of  the  Laramie  is  correspondingly  thinner 
than  is  shown  in  the  table. 
STRUCTURE. 
The  coal  fields  here  described  lie  on  the  edge  of  a  broad  structural 
basin  where  the  beds  dip  gently  to  the  east  or  northeast  and  strike 
parallel  to  the  main  uplift  of  the  mountains  to  the  west  and  south- 
west. This  simple,  moderately  inclined  structure  is  interrupted  by 
minor  folds  which  parallel  the  major  structure  and  involve  the  coal- 
bearing  rocks  in  a  series  of  anticlines  and  synclines.  The  anticlines 
are  gently  arched  and  have  the  steeper  side  facing  the  mountains  and 
more  gentle  slopes  toward  the  basin.  The  synclines  form  shallow 
structural  depressions  about  equal  in  extent  to  the  anticlines. 
Erosion  has  acted  unequally  on  different  parts  of  the  folds.  It  has 
cut  away  the  top  of  the  anticlines  and  exposed  Colorado  shale  in  the 
interior,  encircled  by  escarpments  of  the  higher  formations.  The 
synclines  have  been  affected  to  an  equal  extent,  leaving  outliers  of 
coal-bearing  rocks  in  plains  of  Colorado  shale.  The  position  of  the 
outcrop  of  the  coal  beds  in  each  field  is  dependent  chiefly  on  the 
structure. 
To  the  north  of  the  Ccdy  field  the  coal  beds  are  included  in  the 
normally  eastward-dipping  strata.  On  Shoshone  River  they  occupy 
the  east  flank  of  a  small  anticline.  Along  Sage  Creek  the  beds  appear 
in  the  west  limb  of  a  syncline,  pass  around  its  southern  point,  and  after 
encircling  the  Oregon  Basin  outcrop  in  the  west  side  of  Frost  Ridge. 
In  the  Meeteetse  field  the  structure  is  complicated.  The  coal  beds 
are  included  in  normally  dipping  strata  from  the  Meeteetse  Rim 
southeastward  to  Grey  Bull  River  and  across  the  lower  course  of 
Meeteetse  Creek.  The  coal-bearing  rocks  are  included  in  the  Upper 
Buffalo  Basin  anticline,  a  small  uplift  at  Renners  ranch,  and  a  fold 
along  Wood  River.  Coal  beds  are  also  included  in  a  number  of 
structural  depressions,  notably  the  Sunshine  syncline,  the  trough 
crossed  by  Rawhide  Creek,  and  the  syncline  north  of  Renner's  ranch. 
In  the  Grass  Creek  field  the  outcrop  of  the  coal  beds  encompasses 
an  irregular  uplift  extending  from  Dickie's  ranch  to  Ilo.  Beds  of 
coal  are  also  found  in  a  small  syncline  at  Mayfield's  ranch. 
In  the  Gebo  field  a  strike  ridge  locally  variable  in  direction  extends 
westward  from  Bighorn  River  to  the  east  line  of  T.  45  N.,  R.  99  W., 
where  it  swings  to  the  south  and  joins  a  small  anticline  along  Cotton- 
wood Creek.  In  the  eastern  part  of  T.  44  N.,  R.  99  W.,  the  rocks  are 
brought  up  into  a  small  anticline  which  exposes  coal  beds  in  the 
